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Don’t Be Stupid The following presentation describes real attacks on real systems. Please note that most of the attacks described would be considered ILLEGAL if attempted on machines that you do not have explicit permission to test and attack. I assume no responsibility for any actions you perform based on the content of this presentation or subsequent conversations. Please remember this basic guideline: With knowledge comes responsibility.
Disclaimer The content of this presentation represents my personal views and thoughts at the present time. This content is not endorsed by, or representative in any way of my employer nor is it intended to be a view into my work or a reflection on the type of work that I or my group performs. It is simply a hobby and personal interest and should be considered as such.
Credits The vulnerability that we’ll be discussing was initially discovered by C4SS!0 G0M3S ([email protected]) and was published on June 17, 2011. http://www.exploit-db.com/exploits/17539/ James Fitts created a MetaSploit module that I also reviewed while building this module http://www.exploit-db.com/exploits/17540/
Attack Process • Identify target of interest • Identify software/versions being used • Setup local Instance • Fuzz to identify vulnerability • Design/Develop Exploit • Test • Package/Weaponize
CPU Registers (8086) • EIP – program counter or commonly “instruction pointer” – a processor register that indicates where a computer is in its program sequence. • Holds the memory address of (“points to”) the next instruction that would be executed. • Any thoughts on why this specific register is particularly interesting? Content from: http://en.wikipedia.org/wiki/Instruction_pointer
Buffer Overflow • Software accepts input, but doesn’t ensure that it is only as long as supported. • In this case, software accepts a value into the variable A, but the user sends an overly-long string (“excessive”) and overflows the space allocated to A and overwrites the integer previously stored in B Content from: http://en.wikipedia.org/wiki/Buffer_overflow
Fuzzing • Identify points where application or service accepts data • Send varying lengths/types of data until we crash the service and/or overwrite key buffers. • Increase buffer length until no longer successful (identify upper bounds of memory space available for exploit)
Shellcode • Small piece of code used as the payload in the exploitation of a software vulnerability • Name comes from the purpose – usually spawns a shell and performs some action • Often written in assembly code • Types: – “normal”, Staged, Egg-hunt, Omelette Content from: http://en.wikipedia.org/wiki/Shellcode
[BITS 32] mov ebx, 0x00424F52 ; Loads a null-terminated string “ROB” to ebx push ebx ; pushes ebx to the stack mov esi, esp ; saves null-terminated string “ROB” in esi xor eax, eax ; Zero our eax (eax=0) push eax ; Push the fourth parameter (uType) to the stack (value 0) push esi ; Push the third parameter (lpCaption) to the stack (value ROB\00) push esi ; Push the second parameter (lpText) to the stack (value ROB\00) push eax ; Push the first parameter (hWnd) to the stack (value 0) mov eax, 0x7E45058A ; Move the MessageBoxA address in to eax call eax ; Call the MessageBoxA function with all parameters supplied.
Encoding • There are often restrictions as to what data can be sent via the exploit (NULLs, etc.) • Self-extracting (smaller shellcode) • Self-decrypting (avoid IDS signatures) • Tools such as msfencode offer many options.
Bind Shell/Reverse Shell • Bind Shell – Target exposes a shell on a given port – Attacker connects to that port and executes commands – Remote Administration • Reverse Shell – Attacker listens for connections on a given port – Shell code on target connects to attacker and sends a shell – NAT-safe
Bind Shell Target Attacker Code executes on target and exposes a listener on a specific port (i.e. 4444) Attacker connects (Binds) to client ip:4444 Target sends shell to attacker
Reverse Shell Target Attacker Attacker exposes a listener on a specific port (i.e. 4444) Code executes on target and connects to the attacker ip:4444 Target sends shell to attacker
Design The Exploit • Iterate with various malicious buffer sizes to see how much space is available • Locate where within the evil buffer we actually overwrite EIP • Locate where within the evil buffer we can locate our shellcode (pointed to by other register)
Design The Exploit • Select / configure / encode shellcode • Integrate into exploit script (NOP slide, breakpoints, etc) • Identify reusable jump address to consistently move to shellcode • Test with breakpoints • Test in “real world” scenario
Solutions? • Bounds checking is critical! • Fuzz your own applications • Address Space Layout Randomization (ASLR) • Operating System Support – Data Execution Prevention
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